Optional Union fix

[vitalik]

• Fix [BUG] For optional fields using Union (type1 | type2), it raises exception. #1634: Model | None (union type) in Query parameters crashes with AttributeError: 'types.UnionType' object has no attribute 'model_fields'
• Add private _unwrap_union_model() helper to extract the pydantic model from union types before accessing .model_fields
• Fix both crash sites: _model_flatten_map and detect_collection_fields

Based on

PR #1647 by @Nuung minimized structural changes to existing functions.

[Nuung]

I fully agree. It looks much cleaner. thank you!